Terpolymer Donor with Inside Alkyl Substituents on Thiophene π‐Bridges toward Thiazolothiazole A2‐Unit Enables 18.21% Efficiency of Polymer Solar Cells

Abstract PM6 is a widely used D–A copolymer donor in the polymer solar cells (PSCs). Incorporating second electron‐withdrawing (A2) units into PM6 backbone by ternary D–A1–D–A2 random copolymerization strategy is an effective approach to further improve its photovoltaic performance. Here, the authors synthesize the PM6‐based terpolymers by introducing thiazolothiazole as the A2 units connecting with thiophene π‐bridges attaching alkyl substituent towards the A2 unit (PMT‐CT) or towards D‐unit (PMT‐FT), and study the effect of the alkyl substituent position on the photovoltaic performance of them. Two terpolymers PMT‐FT‐10 and PMT‐CT‐10 are obtained by incorporating 10% A2 units in the terpolymers. The film of PMT‐CT‐10 shows slightly up‐shifted highest occupied molecular orbital (HOMO) energy levels while better co‐planar structure than that of PMT‐FT‐10. Meanwhile, the PMT‐CT‐10:Y6 blend film exhibits better molecular packing properties, more proper phase separation and more balanced hole and electron mobilities, which are beneficial to more efficient exciton dissociation, efficient charge transport and weaker bimolecular recombination. Consequently, the PMT‐CT‐10 based PSCs obtain the highest power conversion efficiency of 18.21%. The results indicate that side chain position on the thiophene π‐bridges influence the device performance of the terpolymer donors, and PMT‐CT‐10 is a high efficiency polymer donor for the PSCs.


2,5-bis(5-bromo-3-(2-ethylhexyl)thiophen-2-yl)thiazolo[5,4-d]thiazole (TTz-CT-Br):
TTz-CT (0.94 mmol, 0.5 g) was dissolved in 50 mL CHCl 3 with 100 ml single-port round-bottom flask, and stirred at atmosphere for 20 min. Then, N-Bromosuccinimide (NBS) (2.07 mmol, 0.37 g) was dissolved in DMF and added in one portion. After the solution was stirred at room temperature for 8 h, the mixture solution was poured into water (200 mL) to quench the reaction. The organic phase was extracted with CH 2 Cl 2 and washed by water and brine for three times and after drying over MgSO 4 . The combined organic phase was obtained and the solvent was removed and purified with silica column chromatography using petroleum ether and dichloromethane (v/v=3:1) as eluent, then further purification was carried out by recrystallization from methyl alcohol/dichloromethane. Finally, the pure compound was obtained as needle-like yellow solid.  34,149.98,142.77,134.10,133.89,115.26,39.89,34.56,32.56,28.66,25.71,23.04,14.10,: m/z 687.02 (M + ). S4 BDT-F monomer (0.1 mmol, 94.05 mg) and BDD monomer (0.1 mmol, 76.67 mg) were dissolved in dry toluene (5 mL) and added into a two-necked flask. The solution was flushed with Ar for 20 min and then the Pd(PPh 3 ) 4 (6 mg) was added into the mixtures, the solution was flushed with Ar again for another10 min. The oil bath was gradually heated to 110℃ and the reactant was stirred for 24h at this temperature.

Synthesis of terpolymers
BDT-F monomer (0.1 mmol, 94.05 mg), corresponding BDD and A2 unit monomers with corresponding proportions were added into a two-necked flask and dissolved by dry toluene (5 mL). The solution was flushed with Ar for 20 min and then the Pd(PPh 3 ) 4 (6 mg) was added into the mixtures, the solution was flushed with Ar again for another10 min. Meanwhile, the oil bath was gradually heated to 110℃ and the reactant was stirred for 24h at this temperature. The further purification was carried with the same procedure as described above. Finally, we further dried in a vacuum over night before use, and got the final product as a black solid.

Measurements and instruments
1 H NMR and 13 C NMR spectra were recorded on Bruker Avance 400 MHz NMR spectrometer at room temperature (Shown in Figure S10-S13). The UV-vis absorption spectra were measured on a UH4150 Spectrophotometer (Direct Light Detector). Cyclic voltammogram (CV) measurements were conducted on a Zahner IM6e electrochemical workstation using sample film coated Platinum disk electrode as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the S5 reference electrode in a 0.1 M tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) acetonitrile solution. Fc/Fc + redox couple was used as the inner reference for the calculation of the electronic energy levels of the polymer donor and acceptor materials.

Transient absorption spectroscopy (TAS)
Femtosecond transient absorption spectrometer was composed of a regenerative-amplified Ti: sapphire laser system (Coherent) and Helios pump-probe system (Ultrafast Systems). The regenerative-amplified Ti: sapphire laser system (Legend Elite-1K-HE, center wavelength of 800 nm, pulse duration of 25 fs, pulse energy of 4 mJ, repetition rate of 1 kHz) was seeded with a mode-locked Ti: sapphire laser system (Vitara) and pumped with a Nd: YLF laser (Evolution 30). The output 800 nm fundamental of the amplifier was split into two beam pulses. The main part of the fundamental beam went through the optical parametric amplifiers (TOPAS-C), whose output light was set as the pump light with wavelength of 830 nm and chopped by a mechanical chopper operating at frequency of 500 Hz. A small part of the fundamental beam was introduced into the TA spectrometer in order to generate the probe light. After passing through a motorized optical delay line, the fundamental beam was focused on a sapphire crystal or YAG crystal, which was used to generate the white-light continuum (WLC) probe pulses with wavelength of 430 to 820 nm or 800 to 1600 nm, respectively. The optical path difference between the pump light and the probe light, which is controlled by the motorized optical delay-line, was used to monitor the transient states at different pump-probe delay. A reference beam was split from the WLC in order to correct the pulse-to-pulse fluctuation of the WLC. The pump was spatially and temporally overlapped with the probe beam on the sample.
Excitation energy of the pump pulse was set to 2 μJ/cm 2 to avoid singlet-singlet annihilation. The film samples for TA measurements were prepared by spin coating the corresponding materials on thin quartz plates. The film samples were thermally annealed the same way as the actual device. (1) where J is the current density, ε r is the relative permittivity of the active layer material, ε 0 is the permittivity of free space, μ is the charge (hole or electron) mobility at zero filed, L is the thickness of the active layer, V eff is the effective voltage (V eff =V 0 -V bias ).

GIWAXS characterization
Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were carried out on a Xeuss 2.0 SAXS/WAXS system (Xenocs SA, France) with X-ray wavelength of 1.5418 Å. The samples were prepared on Si substrates using blend solutions identical to those used in devices and irradiated at a fixed angle of 0.3°.

AFM and TEM measurements
The morphologies of the blend films were investigated by atomic force microscopy (AFM, Agilent Technologies, 5500 AFM/SPM System, USA) in contacting mode with a 1 μm or 5 μm scanner. Transmission electron microscope (TEM) measurements were performed in a JEM-2100F.